1. Field of the Invention
The present invention relates to a capstan servo device for use in a video tape recorder (hereinafter referred to as VTR) which reproduces a video signal and so forth at a changed speed from a tape-shaped recording medium such as a magnetic tape.
2. Description of the Prior Art
FIG. 8 is a block diagram of a conventional capstan servo device which includes a capstan motor 1, a frequency generator (hereinafter referred to as FG) 2 for generating a frequency signal proportional to the rotation rate of the capstan motor 1, a speed detector 83 for detecting a speed error e.sub.v by frequency-discriminating the FG signal, a phase compensator 85, a mixer 86, and a driver 10. Such component elements constitute a speed loop for controlling the rotation speed of the capstan motor 1 to maintain the same constant.
Meanwhile another group of component elements, such as a frequency divider 87, a phase detector 84 for detecting a phase error e.sub.p between the demultiplied output signal of the frequency divider 87 and a reference signal (hereinafter referred to as V-SYNC), and a circuit for supplying such phase error e.sub.p to the mixer 86, constitute a phase loop for controlling the rotation phase of the capstan motor 1 and maintaining the same constant.
The motion of a magnetic tape 82 is maintained at a constant speed by such two control loops. And in a recording mode where a switch circuit 88 is connected to a recording (R) side, the capstan motor 1 is rotated in synchronism with the rotation phase of the V-SYNC signal. Meanwhile in a playback mode where the switch circuit 88 is connected to a playback (P) side, the capstan motor 1 is so rotated that a CTL signal, which is read out by a control head 81 from a control track (not shown) on the magnetic tape 82 and serves as a comparison signal, is synchronized in phase with the V-SYNC signal.
It is generally known in the recent capstan servo devices that, in addition to a recording/playback mode executed with constant-speed rotation of a capstan motor, there are also prepared some other operation modes such as a mode to reproduce a still picture by controlling the stop position of a magnetic tape through control of the rotation and stop of the capstan motor, and a mode to perform changed-speed playback by controlling the capstan motor continuously from slow rotation to fast rotation. In view of such circumstances, further precise control is urgently required.
With regard to the still picture playback mode, an exemplary technique is disclosed in Japanese Pat. Laid-open No. Sho 60 (1985)-39382, wherein the precision relative to the stop position of a capstan motor is enhanced by additionally connecting a stop control circuit to the aforementioned speed and phase servo loops.
Now such stop control for a capstan motor will be described below with reference to a block diagram of FIG. 9 and waveform timing charts of FIGS. 10A-10J.
In a state where a switch circuit 96 is connected to its N side in FIG. 9, a capstan motor 1 is rotated at a normal constant speed under control of a known speed-phase servo circuit 91. And when the switch circuit 96 is changed to its S side representing a stop mode, a signal F.sub.a+ of FIG. 10A produced from an FG signal f.sub.a is supplied as a stop signal relative to a driver 10. In this case, the capstan motor 1 is so controlled as to stop when the voltage applied to the driver 10 is reduced to zero, and the rotation speed is accelerated by a positive voltage increased from the zero point or is decelerated by a negative voltage decreased therefrom. Accordingly, in case a zero crossing point is in each of right slope portions P and left slope portions Q as shown in FIG. 10A, a stable point adapted for stop control is merely the point P while the stability is attained at any point Q, and desired stability is achieved with a shift to either the anterior or posterior point P.
Referring back to the circuit configuration of FIG. 9, the FG 2 generates two-phase signals f.sub.a and f.sub.b mutually having a 90.degree. phase difference. Such two-phase signals f.sub.a and f.sub.b are supplied respectively to non-inverting amplifiers 3a, 3b and inverting amplifiers 4a, 4b of a stop control circuit 92 to become four-phase signals F.sub.a+, F.sub.a- and F.sub.b+, F.sub.b- shown in FIGS. 10A through 10D. And simultaneously the two-phase signals f.sub.a, f.sub.b are supplied also to comparators 93a, 93b to be converted into pulses a t the average-level zero crossing points, whereby rectangular signals Sa, Sb of FIGS. 10E and 10F are obtained. The four-phase signals F.sub.a+, F.sub.a- and F.sub.b+, F.sub.b- are supplied to switch circuits 94a and 94b respectively and are thereby switched in accordance with the rectangular pulses S.sub.a and Sb, so that merely the right slope portions alone are extracted to produce signals D.sub.a and D.sub.b of waveforms shown in FIGS. 10G and 10H. Thereafter such signals Da and Db are mixed with each other in a mixer 95 to become a signal D.sub.s of FIG. 10J where the zero crossing points in the former four-phase waveforms are in the right slope portions alone.
If the signal D.sub.s thus obtained is fed as a stop control voltage to the driver 10 in place of the aforementioned signal F.sub.a+, then the stop of the capstan motor is controlled at the zero crossing points R1-R4 shown in FIG. 10J, so that the stop position accuracy is rendered four times higher than the value in the aforementioned case of using the signal F.sub.a+ as a stop control voltage.
By the employment of such stop control circuit disclosed in the cited Japanese patent, it becomes possible to control the stop position of a magnetic tape with an enhanced four-fold higher precision without the necessity of increasing the frequency of the FG.
In the conventional capstan motor control system where the speed loop based on the FG signal is included in the phase loop based on the V-SYNC signal and the CTL signal as shown in the block diagram of FIG. 8, it is possible to raise the rotation phase precision in conformity with an increase in the gain of the phase loop, and also to reduce, with respect to any disturbance to the capstan motor torque, the harmful influence on the rotational variations in conformity with an increase in the gain of the speed loop and further with a raise of the frequency characteristic. However, there exists a compromise in the gain balance between the phase loop and the speed loop, and if the gain of either loop is increased, resultant mutual interference thereof causes some problems including instability of the loops and a prolonged pull-in time for synchronization. Furthermore, since the linearity of the speed detector does not extent from a low frequency range to a high frequency range, another problem arises with regard to failure in achieving stable rotation speed control from slow rotation to fast rotation. In the method of controlling the capstan motor by additionally connecting the stop control circuit of the cited Japanese patent and switching the same selectively in relation to another stop control circuit separate from the closed servo loop, it is difficult to perform rapid and stable switching exactly from the closed servo loop to the stop control circuit, to eventually fail in attaining satisfactory characteristics.